As used herein, the term "external cavity micro laser apparatus" means apparatus comprising multi-mode micro laser means (as herein defined) having an external cavity for accomplishing mode selection, mode mixing, frequency selection, pulse shaping, beam take-off, and the like.
As used herein, the term "multi-mode micro laser" (or "multi-mode micro laser means") is intended to mean lasing devices, typically but not necessarily of semiconductor construction, which are micro-miniature in size with dimensions typically measured in microns, which may produce one-dimensional or two-dimensional coherent, partially coherent or incoherent emissions, and which produce multiple modes each with multiple lasing lobe components. The term is intended to embrace what are today commonly known as "broad area lasers" or "BALs" which may have an aspect ratio of, e.g., 50:1 to 400:1 (slow axis to fast axis ratio). The term encompasses "laser arrays" which comprise a series of spaced coupled or uncoupled emitters--either broad area lasers or standard lasers. The term also includes laser bars which may be up to a few centimeters wide, e.g., which may contain an array of uncoupled BALs, or a two-dimensional stack of such laser bars. Typical broad area lasers have a single broad stripe for increased output power. Laser arrays have individual current stripes, one for each emitter, which may be closely spaced such that there is a strong mutual coupling or interaction between the light generated by the emitters. In practice, a laser array behaves similar to a broad area laser with respect to its modal properties, except that a laser array prefers to oscillate in higher order modes of order N, where N is equal to the number of stripes or emitters. In a working system "N", for example, might have a value of 10.
An intense need exists for diffraction limited laser sources of several hundred milliwatts of output power for pumping optical fiber amplifiers in communication networks. Commercially available semiconductor lasers are capable of delivering high power, however, the need for an efficient and inexpensive means for coupling the semiconductor laser energy over a few hundred milliwatts into the input aperture of an optical fiber or other optical waveguide has not, prior to this invention, been satisfied.
There are two characteristics of output beams from micro lasers that make single mode fiber coupling inefficient. First, in the slow axis direction (major axis direction of the near field elliptic output beam), micro lasers support multiple transverse modes that are incoherent with respect to each other. Consequently, the output beam cannot be focused with near-diffraction-limited performance in this direction.
Second, the high ellipticity or high aspect ratio of the output beam cross section (typically greater than 1:100 at the near field) results in poor mode matching with the typically circularly symmetric modes of optical fibers.